1. Field of the Invention
The present invention relates to an optical disc apparatus which optically records a signal on an optical disc by means of a light source such as a laser, or reproduces a signal from the optical disc, and particularly relates to the optical disc apparatus which performs focus control where a focus on the optical disc of a light beam is controlled. The present invention further relates to optical information equipment with such optical disc apparatus, and a focus pull-in control LSI provided in the optical disc apparatus.
2. Description of the Related Art
In order to optically record or reproduce information on or from an information carrier by means of an optical source such as a laser, it is necessary to perform focus control such that an information recording surface of the optical disc is constantly located in a focus (converging point) position of a light beam. For realizing this, a so-called focus pull-in operation is performed where an objective lens is moved to bring the focus position of the light beam to the information recording surface of the optical disc prior to the focus control.
Further, in recent optical disc apparatuses, at the request for its increased capacity and reduced thickness, the necessity has arisen for narrowing a distance between the optical disc and the objective lens, a so-called working distance (hereinafter also referred to as WD).
First, narrowing the WD of the objective lens is most effective for attempting to reduce the apparatus thickness. This is because narrowing the WD of the objective lens can not only simply narrow the distance between the optical disc and the lens but also reduce an aperture of the lens as well as a diameter of a raised mirror, and hence the apparatus thickness can be reduced on a further larger scale than the narrowed amount of the WD itself.
Further, a resolution limit is required to be raised with the aim of increasing a recording density for attempting to increase the capacity of the optical disc, which requires an increase in numerical aperture of the objective lens. This leads to an extremely small WD.
As a result of this, there has been a risk in the conventional optical disc apparatuses that at the time of a focus pull-in operation, the objective lens collides with a disc surface of the optical disc to cause damage to the optical disc or the lens system.
Therefore, a variety of measures have been contrived for solving the problem of the collision at the time of the focus pull-in operation.
For example, a conventional optical disc apparatus shown in Japanese Patent Laid-Open Application No. 2003-91833 is provided with a non-contact sensor capable of detecting vertical vibration of the optical disc, and drives an objective lens actuator such that the objective lens gradually approaches the optical disc while making almost the same movement as the vertical movement of the optical disc, to conduct a focus search. It is thereby possible to realize the focus pull-in in which the disc and the objective lens do not collide even in the condition of the narrow WD.
Further, in a conventional optical disc apparatus shown in Japanese Patent Laid-Open Application No. 2002-230792, at the time of pulling a focus on the information recording surface of the optical disc, after a focus servo has once been pulled in the optical disc surface, the objective lens is jump-controlled to pull the focus servo on the information recording surface of the optical disc. It is thereby possible to increase a distance corresponding to a thickness of the substrate of the optical disc as a WD margin at the time of the pull-in of the focus servo, so as to prevent collision between the objective lens and the recording medium.
However, according to Japanese Patent Laid-Open Application No. 2003-91833, the non-contact sensor is required to be separately provided in the optical head device, resulting in increased cost and degraded salability of the product.
Further, the non-contact sensor is required to be installed in a position apart from the objective lens, and hence displacement of the installation position brings about a surface wobbling amount detection error with respect to the objective lens. For example, assuming that the non-contact sensor is installed several tens mm apart from the objective lens in a radial direction of the optical disc, an error of as large as the order of 150 μm is generated with respect to a surface wobbling amount of 300 μm. Or, assuming that the non-contact sensor is installed in a position several tens mm apart from the objective lens in a tangential line direction of the optical disc, the maximum amplitude amount of surface wobble of the optical disc can be detected with almost no error, but phase displacement of a positional variation period occurs, with a phase displacement amount of the order of 40 deg. When this phase displacement is converted into a positional displacement of the optical disc on the middle periphery thereof, the converted displacement amount is of the order of 90 μm. Therefore, since the installment position of the non-contact sensor is apart from the objective lens, the vertical movement detection error due to the surface wobble of the optical disc is large. There thus is a possibility that the optical disc and the objective lens collide at the time of the focus pull-in operation.
Further, individual variation of the non-contact sensors and sensitivity variations of the same due to an installation error are large. As a result, the vertical movement detection error increases due to the surface wobble of the optical disc, which may lead to collision between the optical disc and the objective lens at the time of the focus pull-in operation.
Moreover, according to Japanese Patent Laid-Open Application No. 2002-230792, after the focus servo has once been pulled in the disc surface of the optical disc, the focus of the light beam is jumped to the information recording surface by the jumping operation.
Here, a relative speed of the disc and the lens at the time of switching the servo is called a rushing speed, and a speed range in which a focus detection range is not exceeded is called a rushing speed limit in a case where the objective lens is decelerated at the maximum in the focus servo process.
It is in the case of the rushing speed being within the rushing speed limit depending upon a focus error detection range that the pull-in of the focus servo is safely performed in switching the focus servo process.
In the case of a normal optical disc, the focus error detection range is of the order of 20 μmPP, whereas in the case of a high-density two-layered disc or the like, the focus error detection range is limited to the order of 5 μm PP. This causes the danger of exceeding the rushing speed limit even at a normal rotational speed.
Therefore, according to Japanese Patent Laid-Open Application No. 2002-230792, the relative speed of the disc and the lens immediately after the switch to the jumping operation should at least be a speed of a value obtained by adding a jumping speed and a surface wobbling speed. In addition to this, in the jumping operation, a jump is required to be taken a distance corresponding to the air converted from the thickness of the optical disc substrate. With the jumped distance being long, the relative speed may further increase due to sensitivity variations of the objective lens actuator and power voltage variations. It is therefore thought that in an apparatus where the rushing speed limit decreases, the rushing speed increases and thus easily exceeds the rushing speed limit. Consequently, the disc surface and the objective lens collide, causing the danger of damage to the disc or the objective lens.
Moreover, when the focus pull-in by the jumping operation fails, the objective lens has entered in a wobbling range due to the surface wobble of the optical disc, and the optical disc and the objective lens would almost collide.